feat(svelte): validated opt-in grammar path + all-platform engine checksums

- Build tree-sitter-svelte in one cc command (prebuilt parser.c, no tree-sitter CLI).
- sgconfig.svelte.yml: registers .svelte + injects JS/TS into <script> blocks so the
  existing TS/JS rule packs apply there; rules-svelte/ adds template rules (on:, slot).
  Proven on fixtures/svelte/Messy.svelte; default sgconfig.yml unaffected.
- Record sha256 for all 5 platform ast-grep binaries so first-run fetch is verifiable
  everywhere (binaries stay gitignored — no repo bloat).
- Retire non-working svelte .example stubs; runes migration stays model-driven.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

sgconfig.svelte.yml

@@ -0,0 +1,28 @@
+# Svelte-enabled ast-grep config — OPT-IN.
+#
+# Use this instead of sgconfig.yml ONLY when the Svelte grammar is built at
+# grammars/svelte.so (.dylib/.dll). Build (one-time, ~1s, needs a C compiler):
+#   git clone --depth 1 https://github.com/tree-sitter-grammars/tree-sitter-svelte /tmp/tss
+#   cc -shared -fPIC -O2 -I /tmp/tss/src /tmp/tss/src/parser.c /tmp/tss/src/scanner.c \
+#      -o grammars/svelte.so
+# Then scan Svelte files with:  ast-grep scan -c sgconfig.svelte.yml <paths>
+#
+# This loads the normal rule packs PLUS rules-svelte/, registers .svelte, and injects
+# JS/TS into <script> blocks so the existing typescript/javascript rules apply there too.
+ruleDirs:
+  - rules
+  - rules-svelte
+
+customLanguages:
+  svelte:
+    libraryPath: ./grammars/svelte.so
+    extensions: [svelte]
+    expandoChar: _
+
+languageInjections:
+  - hostLanguage: svelte
+    rule: { pattern: '<script lang="ts">$CONTENT</script>' }
+    injected: typescript
+  - hostLanguage: svelte
+    rule: { pattern: "<script>$CONTENT</script>" }
+    injected: javascript